In recent years, in a radio network system, high-density arrangement of small cells to accommodate suddenly increasing mobile traffic has been examined. Since the small cell has a cell radius smaller than that of a macro cell, the number of user terminals that share the same frequency within the cell can be reduced, thereby improving the throughput of each of the user terminals.
On the other hand, high-density arrangement of small cells increases interference power from an adjacent cell. Assume, for example, that a plurality of cells simultaneously transmit downlink data to different user terminals using the same frequency band. For each user terminal, transmission signals from cells other than a cell that transmits the downlink data destined for the user terminal act as interference power to a desired reception signal, thereby unwantedly decreasing the throughput. To solve this problem, cooperative transmission of the cells is required to suppress interference particularly in a downlink that requires high throughput (non-patent literature 1).
FIG. 20 shows an arrangement formed from one CU (Central Unit) and a plurality of RRUs (Remote Radio Units). At least one or more RRUs are installed in each cell and connected to the one CU via optical fibers (non-patent literature 2). Especially, an optical fiber network that connects the CU and the RRUs is generally called MFH (Mobile Front-Haul).
The CU includes a radio scheduler for centrally allocating the radio resources of the respective RRUs for cooperative transmission of the RRUs. Each RRU has, for example, at least a layer-1 function responsible for signal processing such as modulation/demodulation.
As shown in FIG. 21, the CU performs scheduling for collectively allocating the radio resources of the RRUs to radio transmission of downlink data to perform radio transmission, from the respective RRUs, of the downlink data destined for the UEs received from MBH (Mobile Back-Haul) as a host network. Upon completion of scheduling, the CU transfers each downlink data to the RRU serving as a radio transmission source via the MFH based on an allocation result. The transferred downlink data undergoes baseband processing (layer-1 processing) in the RRU, and is then transmitted to the corresponding UE. When transferring each downlink data to the RRU via the MFH, the CU encapsulates the data based on, for example, the Ethernet® standard, and then transfers it.
In the downlink data transfer processing, the radio transmission destination UEs and radio transmission data amounts of all the RRUs are determined by scheduling in the CU. Consequently, the downlink data to be transferred and their data amounts are not determined before completion of scheduling, and the CU needs to wait for completion of scheduling to start transfer of the downlink data to each RRU, as shown in FIG. 21. On the other hand, transfer of the downlink data to each RRU needs to be completed before the start time of baseband processing in the RRU. This is because resource mapping as part of the baseband processing requires all downlink data whose radio transmission source is the RRU.
Therefore, a period that can be used for downlink data transfer via the MFH, that is, an MFH transferable period is limited to a period from completion of scheduling to the start of the baseband processing. Thus, to economize the MFH by sharing an apparatus or optical fiber core, in arrangements (non-patent literatures 3 and 4) in which a plurality of RRUs are multiplexed by TDM using TDM-PON (Time Division Multiplexing-Passive Optical Network) or a line concentration switch, the MFH transferable period is time-divisionally used by all the multiplexed RRUs, and thus the MFH transferable period of each RRU is shorter, thereby imposing a problem that there may be downlink data which cannot be transferred from the CU to the RRU.